This section provides background information related to the present disclosure which is not necessarily prior art.
There is a presently a strong interest in improving the economics of a water desalination system. Water desalination is expected to continue growing in importance over the coming years as shortages of fresh water continue to be experienced in various regions around the world, and especially in those regions bordering bodies of salt water.
Capacitive deionization is one method of desalination which removes salt from water by using electric fields. However, due to finite charge capacity, the capacitors used in such systems have to be charged and discharged in a continuous cycle of desalination and regeneration. This is done optimally by transferring the charge between two capacitors, and only adding additional energy to the system as energy is resistively dissipated. In a typical capacitive deionization system, the desalination (removal of ions from the water) takes place when the capacitor is charging, and the regeneration (removal of salt from the capacitor) takes place when the electrode is discharging. While there are capacitive desalination systems in which the charge states are reversed, the charging step will be called the desalination step, and the discharging step will be called the regeneration step throughout this document.
The conventional means of doing this transfer is by using a buck boost converter. Such a circuit is shown in FIG. 1. This circuit transfers energy from one electrode (C1) to an inductor (L1), then from the inductor to another electrode (C2). This transfer is done in two steps, and current passes through each electrode only half the time. Electrode 1 (capacitor C1) discharges when electronically controlled switch M3, in the form of a FET, is on (conducting) and electronically controlled switch M1, also in the form of a FET, is off (non-conducting). Current flows into the inductor (L1) and the inductor begins storing the energy from electrode 1 (C1) in its magnetic field. When the current through the inductor reaches a predetermined maximum, then electronically controlled switch M3 is turned off and electronically controlled switch M1 is turned on. At this point the inductor begins transferring its magnetic field energy through electronically controlled switch M1 to the electrode 2 (capacitor C2). When the current flow in the inductor reaches a predetermined minimum, electronically controlled switch M1 is turned off and electronically controlled switch M3 is turned on, and the above cycle repeats itself. It takes many cycles to transfer all of the energy from capacitor C1 to capacitor C2. As such, each one of the electrodes 1 and 2 is only charging or discharging one half the time during any given charging or discharging cycle. This means that it takes twice as long to fully charge an electrode than it would if the charging process was continuous at the same average current. It also means that it takes twice as long to fully discharge one of the electrodes than it would if the discharging process was continuous. One method to speed up the transfer of charge from electrode 1 to the inductor, and then from the inductor to electrode 2, is to allow a greater current flow during the charge transfer process. Thus, using a higher current flow will improve the throughput of the system, but increasing the magnitude of current flow will also increase the power dissipation due to the internal resistances of the electrodes and cables.
In view of the foregoing, any system or method that is able to decrease the total charge transfer time without increasing the power dissipation experienced by the system would significantly improve the efficiency of a capacitive desalinization system.